In addition to allowing a rotorcraft to be maintained in a stable position or to slide on the ground, the undercarriage or landing gear has as its main function that of dissipating at least part of the energy of the impact that is caused by the rotorcraft landing.
This energy is absorbed by friction against the ground, by deformation of the undercarriage, and of its connections with the airframe of the rotorcraft, and where appropriate by shock absorbers. Such deformation is made possible by the parts of the undercarriage being elastic.
The rotorcraft standing on its undercarriage can be thought of as a suspended weight. The resulting mechanical system presents resonant modes of deformation that correspond to resonant frequencies of deformation.
The engine(s) of the rotorcraft, its rotor(s), and the mechanical members for transmitting drive from the engine(s) to the rotor(s), all constitute mechanical excitation sources that produce vibration.
Such vibration can excite said mechanical system in the event of the frequency of the vibration that is produced being close to, or equal to, one of the resonant frequencies of the system: such a situation is referred to as “coupling”.
The term “ground resonance” is commonly used to designate instability of a rotorcraft resting on the ground via its undercarriage that can occur during acceleration or deceleration of the main rotor, or while the rotor is close to its nominal speed, and the helicopter is on the ground. This instability can appear at one or more frequencies close to the first (i.e. the lowest) resonant frequency in rolling (or pitching) of the rotorcraft on its undercarriage, whenever the frequency of rotation of the center of gravity of the main rotor (for providing forward drive and lift) about the axis of the rotor is close to these resonant frequencies.
This particular frequency, written F_excit, is related to the frequency of rotation (Ω) of the rotor and to the frequency of oscillation in drag (ωδ) of the blades of the rotor by the following relationship:F_excit=±Ω+ωδ
The first resonant frequency of the suspension (in rolling or in pitching) of the rotorcraft on its undercarriage depends on several parameters, in particular the stiffness and the damping of the undercarriage structure and of its connections with the airframe (or fuselage) of the rotorcraft, on the relative position of the center of gravity of the rotorcraft and bearing points on the ground, on the weight of the rotorcraft, and on its inertia about the axis under consideration.
These parameters, and the corresponding resonant frequency, can vary considerably for a given rotorcraft, as a function of the equipment with which it is fitted and of its on-board load, and also as a function of various modifications made to a given model of aircraft (of rotorcraft) during its life cycle. When the undercarriage is subcritical, no frequency crossing occurs up to nominal speed, but a change in the weight, in the roll inertia, or in the position of the center of gravity can lead to a frequency crossover occurring close to nominal speed, thereby creating coupling that is explosive. A supercritical undercarriage (lower resonant frequencies) is exposed to frequency crossover during acceleration and deceleration, but any increase in weight or inertia moves these frequencies downwards away from the nominal speed. This thus constitutes a solution that is more robust in the face of changes than that provided by a subcritical undercarriage.
With a supercritical skid undercarriage, it is desired that the frequency of the first mode in roll ωx remains less than the absolute value of the difference between nominal Ω and ωδ, which can be written:ωx<|Ω−ωδ|
It is essential to master these phenomena in order to avoid accidents when the rotorcraft is landing or taking off.
Various skid undercarriage configurations have been proposed in attempts to satisfy those constraints, in particular as described in the following patents: FR 1 578 594 and GB 1 205 263, FR 2 372 081 and U.S. Pat. Nos. 4,196,878, 2,641,423, 3,716,208, 4,270,711, 4,519,559, and EP 113 616.
The undercarriages described in those documents are relatively complex and heavy. The undercarriages having ball joints, springs, or dampers, are difficult to modify over time in order to improve the performance of a rotorcraft.
Another unsolved problem is that of providing a skid undercarriage that is relatively unsophisticated while nevertheless reducing the load factor imposed on passengers during landing, as compared with rigid undercarriages.